One known type of casting die assembly is made up from a fixed die and a movable die, each having a thick die block with a recess defined therein, and a cavity die that is fitted in the recess (see FIG. 5 of Japanese Laid-Open Patent Publication No. 61-226159). The casting die assembly is highly costly to manufacture because it is necessary to bore the recess highly precisely. The casting die assembly also is problematic in that it is heavy, since the die block with the recess defined therein is thick. Further, the casting die assembly is not versatile in use, due to the fact that the cavity die and the recess must be complementary in shape to each other.
To solve the aforementioned problems, there has been proposed a casting die assembly including dies, each of which has a flat attachment surface on which the rear surface of a cavity die is mounted (see, for example, Japanese Laid-Open Patent Publication No. 61-226159). Since the dies of the proposed casting die assembly do not require a recess for a cavity die to be fitted therein, the casting die assembly is relatively low in cost and weight, and is highly versatile in use.